Acoustic ink printing is a promising direct marking technology featuring the important advantage of not requiring the nozzles or the small ejection orifices which have caused many of the reliability and pixel (i.e., picture element) placement accuracy problems that have tended to degrade the performance of conventional drop on demand and continuous stream ink jet printers.
At least two fundamentally different acoustic ink printing techniques have been proposed, so it should be understood that the one to which this invention most directly pertains involves the use of focused ultrasound for ejecting individual droplets of ink on command from the free surface (i.e., the liquid/air interface) of a pool of liquid ink, such that those droplets deposit in an image configuration on a nearby recording medium. It already has been found that acoustic ink printers having printheads comprising acoustically illuminated spherical focusing lenses can print precisely positioned pixels at resolutions which are sufficient for high quality printing of relatively complex images. See, for example, the copending and commonly assigned United States patent applications of Elrod et al, which were filed Dec. 19, 1986 under Ser. Nos. 944,490, now U.S. Pat. No. 4,751,529, 944,698, now U.S. Pat. No. 4,751,530, and 944,701 on "Microlenses for Acoustic Printing", "Acoustic Lens Arrays for Ink Printing" and "Sparse Arrays for Acoustic Printing", respectively. It also has been discovered that the size of the individual pixels that are printed by such a printer can be varied over a significant range during operation, thereby facilitating, for example, the printing of variably shaded images. See, another copending and commonly assigned United States patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,286 on "Variable Spot Size Acoustic Printing".
Although droplet ejectors with acoustic focusing lenses currently are favored for this type of acoustic ink printing, alternatives are available; including (1) piezoelectric shell transducers, such as described in Lovelady et al U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a "Liquid Drop Emitter," and (2) planar piezoelectric transducers having concentric interdigitated electrodes (IDT's), such as described in a copending and commonly assigned Quate et al United States patent application, which was filed Jan. 5, 1987 under Ser. No. 946,682 now U.S. Pat. No. 4,697,195, on "Nozzleless Liquid Droplet Ejectors" as a continuation of application Ser. No. 776,291 filed Sept. 16, 1985, now abandoned. Furthermore, the existing droplet ejector technology is sufficient for designing diversely configured printheads, including (1) single ejector embodiments for raster scan printing, (2) matrix configured ejector arrays for matrix printing, and (3) several different types of pagewidth ejector arrays, ranging from (i) single row, sparse arrays for hybrid forms of parallel/serial printing to (ii) multiple row, staggered arrays with individual ejectors for each of the pixel positions or addresses within a pagewidth image field (i.e., single ejector/pixel/line) for ordinary line printing. Practical considerations can influence or even govern the choice of droplet ejectors for some printhead configurations, so the above-identified patent applications are hereby incorporated by reference to supplement this general overview.
Each of the droplet ejectors of an acoustic ink printer of the foregoing type launches a converging acoustic beam into a pool of liquid ink, typically at a near normal angle of incidence with respect to the free surface of the ink. As a general rule, the angular convergence of each beam is selected to cause it to come to focus essentially on the free ink surface. Furthermore, provision is made in such printers for modulating the radiation pressure which each beam exerts against the free surface of the ink, thereby enabling the radiation pressure produced by each beam to make brief, controlled excursions to a sufficiently high pressure level to overcome the restraining force of surface tension. As a result of these pressure excursions, individual droplets of ink are ejected from the free ink surface on command with sufficient velocity to deposit them on a nearby recording medium.
Typically, such droplet ejectors are driven by rd excited piezoelectric transducers, so the radiation pressures exerted by the acoustic beams they emit can be modulated by amplitude modulating the rf excitation voltages that are applied to their transducers. Unfortunately, however, known rf amplitude modulators generally are relatively complex and expensive devices because of the high frequency response that is required of them. Thus, there is a need for simpler and less costly rf amplitude modulators for use with acoustic ink printheads, as well as for other applications requiring the amplitude modulation of similar rf signals. For example, a basic performance specification for an amplitude modulator intended to be used in a simple acoustic ink printer might call for the modulation of a rf signal having a frequency on the order of 100-200 MHZ and a peak-to-peak voltage of about 100 volts at a peak modulation rate on the order of 100 KHz to vary the peak-to-peak amplitude of the rf voltage by about 2-10 dB as a function of the input data applied to the printer.